A rechargeable battery has become more and more important nowadays. In general a mercury cell has small volume with small capacity. Such a cell cannot be recharged for continuous use. A nickel/cadmium battery also has small volume while being easily rechargeable with longer usable life. However, such a battery is expensive and cannot be so conveniently carried. Moreover, although such a battery has greater capacity than a general cell, it is still insufficient for a great energy-consuming appliance or for long term use and therefore the exhausted battery needs to be replaced frequently causing inconvenience.
Therefore, a lead battery is used to replace the nickel/cadmium battery. The lead battery has a capacity several times that of a nickel/cadmium battery. However, the feature of the saturated voltage of the lead battery is different from that of a nickel/cadmium battery in that the saturated voltage of the lead battery will not gradually lower like that of a nickel/cadmium battery so that a lead battery 10 cannot be properly charged by a charger 12 used for a nickel/cadmium battery 11 as shown in FIG. 1 because the lead battery cannot engage with the charger 12 and the saturated voltage of the lead battery cannot be properly controlled. The lead battery might be taken off when not yet saturated or the lead battery might be over-charged and so heated as to cause damage. Therefore, another charger for a lead battery is required and this is money-wasting and troublesome during travel.
Moreover, the saturated voltage of a nickel/cadmium battery is not constant when charged and greatly varies due to the effect of battery capacity, remaining energy, charging current, temperature, etc. Therefore, we cannot judge whether such a battery is saturated just by its voltage reading. As a result, the manufacturers add a certain kind of "prescription" into the material of the nickel/cadmium battery when manufacturing it so that when the temperature rises, the battery voltage goes down. Therefore, when the saturation is not yet reached, most of the electric power is converted into the potential energy of the battery and thermal energy is hardly produced. While, when the battery is saturated, the electric power cannot be further converted into potential energy and becomes thermal energy to raise the temperature of the battery. At this time, the "prescription" takes effect to lower the battery voltage. The charger senses the gradually lowered voltage and judges that the battery is saturated to stop charging it.
Since the lead battery lacks the above feature that the voltage lowers when saturated and heated, the lead battery is not suitable to be charged by a charger for a nickel/cadmium battery. It is therefore attempted by the applicant to make the lead battery also possess such feature so as to be able to be suitably charged by a charger for the nickel/cadmium battery.